add a 'dist' target to TEMPLATE=subdirs in qmake

Add a rule 'dist' to qmake, so one could
% make dist
on a large project, and it created a tar file that contained all relevant files, from all sub directories.

Right now, qmake doesn't create a 'dist' target for a "SUBDIRS" project:

devin@(none):~/Desktop/code-projects/synthclone$ make dist
make: *** No rule to make target `dist'. Stop.
devin@(none):~/Desktop/code-projects/synthclone$

This is a copy of QTBUG-3124, which was closed because the issue was old, and not because it was resolved in some fashion.